The invention pertains to the field of networks for communications between computers, and, more specifically, to improvements in hubs for such networks.
Networks serve the purpose of connecting many different computers or terminals to each other, host computers, printers, file servers etc. so that expensive computing assets, programs, files and other data may be shared among many users. Communication protocols and standards for networks developed quickly to standardize the way in which data packets were sent across the data exchange media of the network. Several protocols have developed for networks including Ethernet.TM., Token Ring.TM., FOIRL and FDDI, the latter two being adapted for fiber optic physical media carrying the signals.
The physical media first used on Ethernet were thick coaxial cables, and a standard called 10Base5 was developed for assuring multivendor compatibility between components in thick coax, mix and match networks where network components from different vendors were used. These thick coax lines were bulky, expensive and hard to work with. Later, thinner coax Ethernet was developed, and, as an alternative to coax, unshielded twisted pair wires were used for the physical media. A vendor compatibility standard called 10Base-T developed for twisted pair media.
Networks have their own hardware and software to interface with the physical media that carry the signals, and the network software must interface with the operating system software. Computers communicate with each other using a set of rules called a protocol. A group of protocols, all related to the same model are called a protocol suite. To encourage open systems, a common model called OSI was developed by the International Standards Organization. OSI engendered a protocol suite which allows computers of all sizes and capabilities the world over to communicate using a common set of rules.
The OSI model has seven layers of software, each of which makes different functionality available to computers communicating using this model. Each layer in the model deals with specific computer-communication functions.
The Physical Layer is the lowest layer and specifies the rules for transmission of signals across the physical media. Hubs, also known as repeaters, have multiple connections to this physical media called pods. The purpose of a hub is to receive data packets from one port and repeat these packets, i.e., retransmit them on every other port connected to the hub according to whatever protocol, e.g., Ethernet, etc., which is in use.
The Data Link layer deals with transmission of data between devices on the same network. In addition to describing how a device accesses the physical media, this layer also provides some measure of error detection and control. Local Area Network (LAN) technologies such as Ethernet, Token Ring and FDDI operate at this layer. Data link addresses are implemented at this layer, and provide each device connected to the network a unique identifier by which packets may be sent to it. Bridges, which are devices which aid in forwarding data packets from one network segment or one network to another, operate at the Data Link layer.
The Network Layer deals with transfer of data between devices on different networks. The Network Layer adds the notion of network addresses which are specific identifiers for each intermediate network between a data source and a destination. Routers, which are devices which assist in transferring data packets from one network to another, operate at the Network Layer.
The remaining layers, called the higher layers, are the Transport Layer, Session Layer, Presentation Layer and Application Layer. These layers deal with communication between message source and message destination. The transport layer manages the transfer of data from a source program to a destination program. Process addresses, which identify specific "processes", i.e., computer programs, are implemented at this layer. Gateways operate at these higher OSI layers.
Within the OSI model, the user presents data through application programs to the highest layer. This data is then passed downward through the hierarchy of layers with each layer adding addressing and/or control information. When the data reaches the physical layer, it is sent to a device.
Conversely, received data is passed up through the layers with each layer stripping address or control information.
One way to think of a protocol is a common language by which computers may communicate, but a more accurate way is as a set of rules by which data is communicated between identical OSI layers.
There are other communication protocols beside the OSI Model. These include TCP/IP, XNS, IPX, AppleTalk, DECnet and SNA. Each of these protocols has its own layer model. For example, TCP/IP collapses network functionality into only 4 layers, while AppleTalk has 6 layers.
All network media have a limitation on the maximum volume of traffic that may be carried based upon the bandwidth imposed by the physical characteristics of the media. Ethernet bandwidth is 10 Megabits/second. This acts a limit on the traffic volume and can limit the number of computers, which may be connected to a single "segment" of a network. A segment is section of a network connected to a group of machines which may communicate with each other via repeater operations without having to traverse a bridge or router. Bridges and routers are useful in that they allow connections of multiple segments such that more computers may communicate with each other than would otherwise be possible given the limited bandwidth of the media.
Each bridge and router requires certain other peripheral circuitry to support it such as LAN controllers, a CPU, a power supply, a network management process, memory to store bridge source and destination address tables and various other things like status registers etc. Likewise, repeaters require many support circuits many of which are the same support circuits needed by bridges and routers. Further, bridges, routers and repeaters or hubs require initialization to set them up for operations, and they require initial installation labor to set them up properly to operate in a particular network configuration. In addition, each type machine is subject to network management considerations, assuming an intelligent hub. An intelligent hub is one which collects statistics about traffic flow through its ports, can electronically turn ports on and off and which provides error correction and detection services. Intelligent bridges, routers and hubs supply status information upon request from network management processes and can respond to network management commands, such as shut off a particular port.
In the prior art, bridges and routers were separate circuits from hubs and this created needless duplication of many peripheral circuits which were common between hubs and bridges and which could be shared. This needless duplication cost more and provided more points of failure. For example, if the bridge power supply failed or the CPU crashed, all machines on the two network segments on either side of the bridge would be cut off from each other.
Typically, a bridge is connected to a hub by a separate local area network segment which itself requires two port interface circuits such as LAN controllers and AUI's (generic network interfaces) with appropriate port drivers adapted for the specific media used for the bridge-hub LAN segment. This bridge-hub LAN segment represents an additional expense, requires management and provides additional points of failure which could disable the network. An intelligent hub coupled to a bridge or router by a separate LAN segment then requires three different device addresses for management message traffic, and creates more possibility for a network failure in multiplying the number of points of possible failure.
Another drawback of separate bridge/router and hub circuits is that bridge/routers do not usually include a mode where the bridge/routing function can be bypassed. The ability to bypass the bridge/routing function provides flexibility in network growth as small networks do not need bridging functions until the maximum network traffic volume starts to exceed the available network bandwidth. The ability to selectively bypass the bridge/routing function gives a network designer the ability to design a small network which has a built in capacity to grow larger without adding new components and improves the ability to troubleshoot the network.
Integrated hubs and bridges existed as option cards for concentrator chassis at the time this patent application was filed. One example of such a device is the Penril 2530 concentrator card with full performance bridging although it is not currently known whether this device qualifies as prior art because the copyright date of the literature on this device is dated the same month as the filing date of this patent application. The Penril Module 2530 10BaseT concentration and bridging card for the Penril 2500 series concentrator combines a hub and bridge which operates at all times on the same printed circuit board. The design of the Penril 2500 concentrators were for large networks. The 2530 card slides into a card slot on the 2500 series concentrator which can also service a plurality of such cards. The concentrator frame is believed to contain certain shared features such as power supply etc. and has a local, internal LAN segment that couples all the repeater/bridge cards together so that they can send data back and forth between them. The repeater on each card can be coupled to up to 25 machines on the network segment connected to that card and the integrated bridge continuously bridges the network segment coupled to a particular card to the internal LAN segment such that a machine coupled to a LAN segment coupled to card 1 can send a packet to a machine coupled to a LAN segment coupled to card 2 via the bridge on card 1, the internal LAN segment of the concentrator, the bridge on card 2 and the repeater on card 2. No distributed management functionality is integrated on either card 1 or 2. That management functionality is placed on a third card which is plugged into a different card slot. If the management card fails, the repeaters and bridges in cards 1 and 2 cannot be controlled. Likewise, if the internal LAN fails, user 1 cannot send data to user 2 or vice versa.
A concentrator structures like the Penril 2500 series is designed for large networks since to connect two external network segments, two cards are needed each of which can service up to 25 user machines. If the network has only 27 users, such a concentrator represents too big and complex of a structure to be affordable and justifiable.
Another problem with concentrators such as the Penril 2500 series is their lack of "stackability". The problem which prompts the need for a stackable network slice architecture is as follows. Suppose a particular building had 3 users on the ground floor and a group of 20 heavy users on the 4th floor or otherwise spaced away from the 3 users on the ground floor by a distance which is just under the maximum 10BaseT cable run permitted by the applicable Ethernet specification. The use of a concentrator requires that every one of the group of 20 users has his own twisted pair running from his machine back to the concentrator. The same is true for thick and thin coaxial cable installations. Such a configuration can be prohibitively expensive because a great deal of wire or coax must be used and the expense of installing all that wiring through the walls and ceilings can be large.
Now suppose that the distance to the group of 20 from the concentrator is larger than the maximum allowable cable run. In such a case, the complex wiring cannot be used, and if those users must be able to share resources with the 3 users on the ground floor, another concentrator must be purchased. Concentrators like the Penril are expensive. Typical costs today are in the neighborhood of $30,000 for the concentrator frame and about $6000 for each card.
A similar problem arises in large networks in big companies who may, for example, have a branch office in another state with only 6 users. If those users must share data or resources connected to the network at the parent company, they must be on the same network as the users at the parent company. With concentrator technology, the 6 users in the branch office must be connected to the concentrator at the parent company by a wide area network (WAN) connection. The Penril concentrator 2500 series has a card module (the 2540) which implements a WAN interface, but the 6 users in the branch office must also have a concentrator to plug their WAN interface card into. Therefore, the expense of having the tiny 6 user network segment remotely located is greater than it needs to be.
Thus, a need has arisen for an apparatus which can perform the functionality of bridges or routers and hubs without the aforementioned deficiencies, and which can overcome the aforedescribed difficulties with concentrator technology in smaller networks or large network will small satellite networks.